The present invention relates to the protection of power transformers in a power system network in general, and more specifically to the protection of the transformer with a sudden pressure relay system and apparatus for supervision thereof.
Referring to FIG. 1, a power transformer 20 is provided between the line sections 22 and 24 for coupling power therebetween in a power system network. The schematic illustration of FIG. 1 shows only one phase of the power system network, but it is understood that the network may include three phases, for example, in which case there would be an additional line section for each phase. The simple example of FIG. 1 depicts the transformer 20 with only two windings, 26 and 28.
A conventional differential relay unit 30 is provided for protecting the power transformer 20 from internal faults. Currents depicted by the arrows 32 and 34 entering and leaving the transformer 20 through line sections 22 and 24, respectively, may be measured by current transformers 36 and 38 disposed correspondingly on either side of the transformer 20 using the line sections 22 and 24, respectively, as the primary windings thereof. The current transformers 36 and 38 establish a differential zone 40. Also disposed at either end of the transformer 20 between its corresponding current transformer 36 or 38 and the transformer 20 is a conventional breaker unit 42 or 44, accordingly. In general, the differential relay unit 30 monitors the currents 32 and 34 of the power transformer 20 via current transformers 36 and 38 and from this information determines if an internal fault exists within the power transformer 20. In the event that an internal fault is detected, the unit 30 energizes a relay coil 46, denoted as DR, which ultimately causes the breakers 42 and 44 to isolate the power transformer 20 from the power system network. Conventional differential relay units generally also include an implementation which distinguishes external power line faults outside of the differential zone 40 from internal faults of the transformer 20 and cause the relay unit 30 to be inoperative with regard to a detected external fault.
Moreover, in connecting the power transformer 20 to the power system network, the general procedure is to close the breakers 42 and 44 sequentially. When the transformer is excited by the current through the first closed breaker, the initial surge of excitation current upon closure of the first breaker may be very high, on the order of 10 times the rated current of the power transformer 20. Under these conditions, there is a differential current established about the transformer 20 as measured by the current transformers 38 and 36. For example, suppose breaker 44 is closed first, then the current transformer 38 measures the inrush current associated therewith and the current transformer 36 measures zero current because the breaker 42 has not yet been closed. Present differential relaying units 30 are designed to distinguish between inrush and internal fault currents to permit the relay unit 30 to disregard overcurrent conditions which result from excitation inrush.
However, to accomplish this capability of distinguishing between excitation inrush and internal fault overcurrent conditions, the relay overcurrent detection of the differential relay was desensitized to the point where internal turn-to-turn winding fault conditions of the power transformers were undetectable. Consequently, a single turn short in the winding of a power transformer could not be distinguished by a conventional differential relay. Thus, the conventional differential relay units could not be relied upon to fully protect the power transformers within their power system networks.
To complement the differential relay in the protection of the power transformers, a sudden pressure relay (SPR) 50 was introduced into the protection scheme. The operation of the SPR required that the power transformer 20 be enclosed in a sealed tank, denoted in FIG. 1 by the box 48. The SPR 50 is disposed through the sealed tank and is responsive to the rate of change of gas pressure inside the sealed transformer tank 4. Accordingly, the SPR 50 is sensitive enough to respond to a pressure change inside the sealed tank 48 which is caused by turn-to-turn faults. Unfortunately, the SPR 50 will also respond to severe external faults. Upon detection of a faulty condition, the SPR 50 may energize a relay coil SPR 52.
The contacts of the relay coils DR and SPR may be logically combined as shown in the schematic of FIG. 2 to energize a lockout relay coil, denoted as LR. The contacts SPR and DR are connected in parallel between an upper potential denoted as Vu and a normally closed contact LR of the lockout relay coil. The lockout relay coil is coupled between a lower voltage potential V.sub.L and the other side of the normally closed contact LR. Thus, when either of the relay coils DR or SPR is energized, current is allowed to energize the relay coil LR between the upper and lower voltage potentials Vu and V.sub.L, respectively. Once energized, the lockout relay operates to open the breakers 42 and 44 and isolate the transformer 20 from the power system network. Thereafter, the lockout delay coil may only be reset manually by an operator, for example.
One drawback of the aforementioned protective arrangement is that severe external fault conditions may cause false operation of the SPR which results in a degradation of the security of the protection system. More specifically, severe external faults cause heavy bulk currents to flow through the transformer 20 which introduces a severe mechanical vibration from the transformer windings to render a sufficient rate of change of the gas pressure in the sealed tank to operate the SPR 50. Because the SPR 50 cannot distinguish between the rate of change of pressure caused by a through or external fault and a legitimate internal fault, the protective scheme utilizing the SPR is vulnerable to false operations. Since the SPR provides such a necessary and sensitive fault detection function for a utility's most critical and expensive transformers, a solution of this problem is of paramount importance.
One solution to the aforementioned problem is described in the paper "Current Supervision of Fault Pressure Relays on Large EHV Transformers", authored by Grimes, D. E. and Mozina, C. J. from Cleveland Electric Illuminating Company which was presented at the Pennsylvania Electric Association Relay Committee Meeting in Reading, PA, Feb. 24, 1977. The authors' proposal was to make the SPR 50 inoperative for high magnitude faults above the thresholds of operation of the transformer differential relay 30. A simple embodiment of this concept is shown in the schematic illustration of FIG. 3. Grimes and Mozina propose to use an overcurrent relay 60 to supervise the SPR operation. The purpose of the overcurrent relay 60 is to detect an external fault condition and render the SPR 50 inoperative. To achieve this, the overcurrent relay 60, denoted as OCR, may be disposed in series with the current transformer 38, for example. The overcurrent relay 60 may be set higher than the maximum or rated load current of the transformer, thus if a very heavy current is created from either an external or internal fault, the OCR 60 will operate to open a normally closed contact. The overcurrent relay contact OC is added in series with the SPR contact in the relay logic as shown in the relay logic schematic diagram of FIG. 4. In this arrangement, when the OCR 60 is activated, the contact OC opens, thus blocking the circuit path of the SPR contact from energizing the lockout relay LR. One very important requirement for this overcurrent supervision scheme is for the OCR 60 to detect and react to the heavy current condition before the SPR 50 reacts and closes the contact SPR. In addition, the OCR 60 must remain energized for a period of time beyond the time when the heavy current condition is relieved to permit the SPR to stabilize and open the SPR contact.
The aforementioned described solution appears to be adequate for a two winding transformer such as has been described in connection with the embodiment of FIG. 3, but a dilemma is created for transformers of three or more windings, i.e. multiple-winding transformers, in deciding how many OCR's to include and where to dispose the OCR's in the protective circuitry. For example, for a three winding transformer, at least two OCR's will be needed to cover all possibilities of external through-fault conditions. In general, for a transformer having N windings, there will be needed N-1 OCR devices for each phase of the power system network which become quite cumbersome and expensive to implement.
Another drawback of the overcurrent supervision scheme just described is that the OCR does not discriminate between severe overcurrent internal and external fault conditions. This results in blocking the operation of the SPR even under the desirable operating conditions of an internal fault. Consequently, the redundancy feature of paralleling the SPR and DR contacts (see FIG. 4) is eliminated.
The present invention of a supervisory unit for the SPR system proposes to eliminate the need of the OCR's for SPR supervision and overcome the aforementioned drawbacks without loss of protection security for the power transformer.